Focussing arrangement for X-ray tubes



Aug. 4, 1964 J. R. STANSFIELD FOCUSSING ARRANGEMENT FOR X-RAY TUBES Filed June 6, 1960 2 Sheets-Sheet 1 3) Maw;

A g- 1964 J. R. STANSFIELD FOCUSSING ARRANGEMENT FOR X-RAY TUBES 2 Sheets-Sheet 2 Filed June 6, 1960 United States Patent O FOCUSSING ARRANGEMENT FOR X-RAY TUBES James Richard Stansfield, London, England, assignor to Hilger & Watts Limited, London, England, a company of Great Britain and Northern Ireland Filed June 6, 1960, Ser. No. 34,265 Claims priority, application Great Britain June 11, 1959 12 Claims. (Cl. 313-57) This invention relates to X-ray tubes and to cathode assemblies for X-ray tubes and an object of the invention is to provide means which will enable the obtainment on the anode of a line focus, preferably a line focus which has a substantially even electron intensity along its length, while at the same time maintaining a comparatively short distance between cathode and anode. As will hereinafter appear, this is done by means of a novel cathode assembly which can be incorporated in an X-ray tube which is basically of known design.

The existence of X-ray tubes having a line focus at the anode is in itself known (see, for example, United States Patent No. 2,683,233, Hosemann). In this atrangement the cathode takes the form of a coated filament located in an opening in a cylindrical Wehnelt electrode; the required focussing is effected by means of an intermediate electrode and which is provided with a partition extending across the tube between the cathode and anode, this partition forming at its middle part a cupshaped member provided with an aperture which is preferably shaped as a straight slot parallel to the cathode helix. This forms, in effect, a focussing lens for the electron beam. In the described embodiment the voltage between the cathode and the intermediate electrode is of the order of one-fifth of the voltage between the cathode and anode, and the distance between the cathode and the intermediate electrode is between 4 and 8 ems. according to the voltage of the tube.

For certain applications it is desirable to have a much smaller distance than 4 cms. between cathode and anode, thus shortening the distance between the anode and the end of the tube, and allowing the X-rays to issue from a position near the end of the tube. The reduction of distance between cathode and anode to a distance of say 1 cm. necessitates the avoidance of any construction having a focussing lens such as that provided by the Hosemann tube.

This reduction of the distance between cathode and anode can however be achieved in the fine focus X-ray tube developed by Ehrenberg and Spear which gives a focal spot about 40 microns in diameter, and is suitable for crystallographic work. (Proc. Phys. Soc. B, 64, p. 67, 1951, and United Kingdom patent specification No. 701,- 050.) This X-ray tube is of great value for single crystal studies, especially when using very small crystals, and also for certain powder diffraction work where fine specimens and small cameras can be used, and patterns are obtained with high resolution and short exposure time.

In many X-ray tubes, including this particular one, the limit to the operating power is set by the heat produced at the anode by the electron bombardment over the area of the focus. The power may be increased by providing efiicient cooling to the anode and also it is possible to operate at higher power per unit area of focus for a small focus than for a large focus. This is one reason for the satisfactory performance of the Ehrenberg and Spear tube.

There are other occasions when a larger focus is permissible, and a higher total power desirable, and for this purpose a tube has been developed giving a focal area of about 0.1 x 1.4 mm. (Acta Cryst 10, p. 52, December 1957). In such a tube the power per unit focal area is necessarily lower, but the total power higher.

One feature of this X-ray tube is that it is identical with the Ehrenberg and Spear tube except for the cathode assembly (or cathode gun) and that interchangeable assemblies are therefore possible, enabling user to select the required focal area.

In other applications a larger focus again, and a correspondingly higher power is desirable, particularly when electronic methods of detection are used.

It is a feature of the X-ray tube of Ehrenberg and Spear that the electron discharge path from cathode to anode lies within an element which is maintained at a potential in the region of cathode potential, and the present invention relates to X-ray tubes having this feature (hereinafter referred to as X-ray tubes of the kind described). It is also a feature of the Ehrenberg and Spear tube that the element does not have any partition extending inwardly into the tube between the cathode and the anode to form a focussing lens. In practice it is convenient for the said element to be constituted by the wall of the tube which is maintained at earth potential in which case the anode is at a high positive potential with respect to earth and the cathode is at near earth potential.

A practical advantage of X-ray tubes of the kind described is that as the cathode assembly is at nearly the same potential as the element round the path of the electrons between cathode and anode the cathode does not require a high degree of electrical insulation and it can be made compact and easily removed, so that it can be interchanged for another cathode assembly when desired.

One result of the said element being at a potential in the region of cathode potential is that a strongly convergent electrostatic field is produced which in part contributes to the fine point focus originally achieved. If, for the purpose of obtaining a line focus, there is placed into the tube a cathode consisting of a straight filament in a focussing slot (such as is used in known forms of X-ray tube with both tube and anode at earth potential and the cathode highly negative) it is found that no satisfactory line focus results as its length is reduced to an undesirable extent. If, on the other hand, one incorporates an uncoiled cathode filament having an emission surface which is convex towards the intended direction of electron fiow and a metal shield forming a focussing slot for the said filament which is biased at a negative potential relatively to the filament, one can obtain a satisfactory line focus; and by suitably dimensioning the filament and shield having regard to the dimensions of the X-ray and the voltages to be applied one can obtain a line focus having a substantially even electron intensity along its length.

According to the present invention therefore we provide a cathode assembly for an X-ray tube of the kind described having an uncoiled cathode filament with an emission surface convex towards the intended direction of electron flow and a metal shield forming a focussing slot for the said filament and arranged so that it can be biassed at a negative potential relatively to the filament, the edges of the shield adjacent the filament also being convex towards the intended direction of electron flow.

The cathode assembly is preferably so designed, that, when assembled in the X-ray tube of the kind described for which it is intended, and the requisite operating volt ages are applied, there is at the anode a line focus having a substantially even electron intensity along its length.

The cathode assembly of the present invention may be, but is not necessarily, constructed to be easily removable from its X-ray tube so as to be interchangeable with another cathode assembly of the same, or a different, type.

A complete X-ray generator according to the present invention includes an X-ray tube of the kind described having a cathode assembly according to the present in- {3 vention and of course incorporates circuits to provide the necessary potentials for the anode the cathode and the metal shield forming the focussing slot.

One embodiment of the present invention will now be described with reference to the accompanying drawings wherein FIG. 1 is an elevation of a cathode assembly according to the invention;

FIG. 2 is a side sectional elevation of the cathode assembly at right angles to FIG. 1;

FIG. 3 is an end view of the cathode assembly;

FIG. 4 shows, partly in section, an X-ray tube incorporating the said cathode assembly; and

FIG. 5 shows the electric high tension and biassing circuits for the tube.

Referring first to FIGURES 1 to 3 of the drawings the cathode assembly comprises a wire filament and a metal shield 11 both mounted on an insulating base 12 which is intended to form the end cap of an X-ray tube to be described later. The filament is located in a slot 13 2 mm. wide in the shield 11. The filament has two straight leg portions 14 and a central curved portion 15. The ends of the filament are held in posts 18 embedded in the in sulating' base 12 and these in turn are electrically connected to conductors 23 by which current is to be led to the filament 10. The shield 11 is cylindrical in shape, with recesses 21, 22 in each side to leave room for the posts 18 and the top of the shield forms the surface of another cylinder 16 to provide curved edges 17 adjacent the filament of smaller radius than the radius of the curved portion of the filament 10. The centres of the slot edges 17 protrude 1.6 mm. beyond the centre of the curved part 15 of the filament. The shield 11 is mounted in the insulating base 12 and is provided near its base with a duct 24 for cooling Water which is led to and away from the duct by inlet and outlet metal conduits 25 and 26.

It will be appreciated fromthe above description that the conduits 25 and 26 form an electrical connection to the shield 11 and, as they are insulated from the conductors 23 and the filament 10, the conduits 25 and 26 and the conductors 23 can be utilized as means for biassing the metal shield negatively relatively to the filament and also as means for maintaining the wall of an X-ray tube in which the cathode assembly is mounted at a potential in the region of cathode potential.

The cathode assembly above described can be used in various forms of X-ray tubes of which one will now be described with reference to FIGURE 4 of the drawings.

The X-ray tube is mounted on an insulator 27 having an annular groove 28 round which is mounted an externally screwedmetal ring 29. Above the ring 29 is located a tubular lower housing 30 having near its lower end a shoulder 31 against which abuts the mating shoulder of a threaded collar 32 which is screwed on the metal ring 29 in order to compress a flexible O-ring 33 between the lower end of the lower housing 30 and the insulator 27, thus forming a gas-tight seal. 7 7 From one side of the lower housing 30 extends a pipe connection 34 which can be connected to a vacuum pump (not shown) for the purpose of evacuating the casing of the X-ray tube.

Welded to the upper end of the tubular lower housing 30 is an extension tube 35 to the top of which is welded an upper housing 19, which in its turn bears at its upper end the cathode assembly, which is also welded thereto, Near the upper end of the upper housing 19 are alternative windows 36, one on each side, for the X-rays from the tube; a movable shutter 37 outside the part of the upper housing 19 in which the windows 36 are situated screens one or other of the windows 36 and permits X-rays to issue from the other.

' The anode comprises a target 20 which is the upper end of a tubular body 38 which extends downwardly through the insulator 27; the tubular body 38 has a bend 39 near its lower end and terminates in a pipe connection 40. An

4 inner conduit 41 extends upwardly inside the tubular body 38 nearly up to the target 20, this inner conduit terminating at its lower end in a separate pipe connection 42. The pipe connections and 42 can be used for the inlet and outlet of cooling water for the anode.

Round the tubular body 38 is located a supporting-insulator 43 which has radially extending members 44 of which one is shown, for the purpose of locating the tubular body 38 inside the X-ray tube. The arrangement is such that the target 20 is at a distance of 4.5 mm. from the nearest part of the metal shield 11 of the cathode assembly, and the axis of the X-ray tube passes through the middle of the curved portion 15 of the filament10.

A circuit diagram for the high tension and biassing circuits is shown in FIGURE 5. The filament 10 is connected across the secondary winding 51 of a filament transformer 50 connected on its primary side to a suitable source of alternating voltage. From a tapping 52 on the secondary winding 51 a conductor leads to a resistor 53 the other end of which is earthed. A movable tapping on the resistor 53 is also earthed as is the wall of the X-ray tube comprising the upper housing 19 the extension tube 35 and the lower housing 30. The metal shield 11 is also earthed; this may be effected by electrically connecting one or other of the inlet and outlet conduits 25 and 26 to the upper housing 19.

The anode of the X-ray tube is connected, as usual, to a suitable high voltage relative to earth.

The above-described circuit provides a small positive potential for the filament 10, the value of which can be adjusted by varying the position of the movable tapping on the resistor 53. The combined effect of the curved filament and the curvature of the edges 17, coupled with the negative bias for the shield 11 (relatively to the filament), is to enable a line focus to be obtained some 5 or 6 mm. in length and a few tenths of a mum. in breadth with a substantially even electron intensity throughout its length. The curvature of the filament plays an im portant part in achieving this even loading along the focus; too much curvature results in high intensity at the centre, fallingoff to each end; too little curvature gives undesirable end loading. a

With the arrangement shown in the drawings, provided adequate cooling is applied to the anode, powers of up to 450 watts (50 kv., 9 ma.) may be dissipated.

In order to obtain the maximum possible output from the X-ray tube it is operated close to the limit above which the target pits. As a consequence, the target surface of the anode should be re-worked at relatively frequent intervals, depending upon how close to the upper limit of power it is operated. For moderate powers (up to or 200 Watts) a detachable target tip 20 may be used; at higher powers it is desirable for the target to be integral with the cooled anode stem so as to avoid the thermal resistance of an interface.

What I claim and desire to secure by Letters Patent is:

1. A cathode assembly, for an X-ray tube of the kind in which the electron path from cathode to anode lies within an element which is maintained at a potential in the region of cathode potential, having a filament having an emission surface convex towards the intended direction of electron flow and a metal shield forming a focussing slot for the said filament and arranged so that it can be biassed at a negative potential relative to the filament, the edges of the shield adjacent the filament also being convex towards the intended direction of electron flow, said shield being positioned with the slot aligned with the filament.

2. A cathode assembly according to claim 1 wherein the said filament forms the arc of a circle and the adjacent edges of the shield form arcs of smaller radius.

3. In an X-ray tube an anode, a filament with an emission surface convex towards the anode, the said filament and anode providing a path for electrons between the filament and the anode, a metal shield forming a focussing slot for the said filament with the forward edges of the shield adjacent the filament convex towards the anode, said shield being positioned with the slot aligned with the filament, an element round the electron path, means for maintaining the said element at a potential in the region of filament potential and means for biassing said metal shield at a negative potential relative to the filament.

4. In an X-ray tube an anode, a slotted metal shield with the forward edges of the shield defining the slot convex towards the anode, a filament having an emission surface convex towards the anode and located within the slot in the shield, said shield being positioned with the slot aligned with the filament, an element round at least part of the space between the filament and anode, means for maintaining the said element at a potential in the region of cathode potential and means for biassing said metal shield at a negative potential relative to the filament whereby the electrons from said filament follow a path which forms at the anode a line focus having a substantially even electron intensity along the length of the line.

5. In an X-ray tube an anode, a slotted metal shield with the forward edges of the shield defining the slot forming similar arcs of a circle convex towards the anode, said shield being positioned with the slot aligned with the filament and anode, a filament located Within the slot in the shield and with an emission surface forming another are convex towards the anode and of larger radius than the aforementioned arcs, an element round at least part of the space between the filament and anode, means for maintaining the said element at a potential in the region of filament potential and means for biassing said metal shield at a negative potential relative to the filament.

6. An X-ray generator incorporating an Xray tube having an anode and a filament with an emission surface convex towards the anode the said filament and anode providing a path for electrons between the filament and the anode, a metal shield forming a focussing slot for the said filament with the edges of the shield adjacent the filament convex towards the anode, said shield being positioned with the axis of the anode passing through said slot to said filament, an element round the electron path and electric circuits to provide an operating potential difierence between cathode filament and anode, to hold the element round the electron discharge path at a potential in the region of filament potential, and to hold the said metal shield at a negative potential relative to the filament.

7. An X-ray generator incorporating an X-ray tube having an anode, a slotted metal shield with the forward edges of the shield defining the slot convex towards the anode, a filament having an emission surface convex towards the anode and located within the slot in the shield, said shield being positioned with the axis of the anode passing through said slot to said filament, an element round at least part of the space between the filament and anode and electric circuits to provide an operating potential diiference between filament and anode, to hold the said elei rent at a potential in the region of filament potential and to hold the said metal shield at a negative potential relative to the filament, the electrons from said cathode forming at the anode a line focus with a substantially even electron intensity along the length of said line.

8. An X-ray generator incorporating an X-ray tube having an anode, a slotted metal shield with the forward edges of the shield defining the slot forming similar arcs of a circle convex towards the anode, a filament located within the slot in the shield and having an emission surface forming another are convex towards the anode and of larger radius than the aforementioned arcs, said shield being positioned with the axis of the anode passing through the middle of said slot to said filament, an element around at least part of the space between the filament and anode and electric circuits to provide an operating potential difierence between filament and 5 anode, to hold the said element at a potential in the region of filament potential and to hold the said metal shield at a negative potential relative to the filament, whereby the electrons from the filament form at the anode a line focus with a substantially even electron intensity along the length of said line.

9. In an X-ray tube, an anode, a filament having an emission suriace convex towards the anode, the said filament and anode providing a path for electrons between the filament and the anode, a metal shield forming a focussing slot for the said filament with the edges of the shield adjacent the filament convex towards the anode, said shield being positioned with the slot aligned with the cathode and anode, an element around the electron path, means for maintaining the said element at a potential in the region of shield potential, said element being cylindrical in form and defining the sole focussing device between the metal shield and the anode, and means for biasing said metal shield at a negative potential elative to the filament.

it). In an X-ray tube, an anode, a slotted metal shield with the forward edges of the shield defining the slot convex towards the anode, a filament having an emission surface convex towards the anode and located within the slot in the shield, said shield being positioned with the slot aligned with the cathode and anode, an element around at least part of the space between the filament and anode, said element being cylindrical in form and defining the sole focussing device between the metal shield and the anode, means for maintaining the said element at a potential in the region of shield potential and means for biasing said metal shield at a negative potential relative to the filament.

11. An X-ray generator incorporating an X-ray tube having an anode and a filament with an emission surface convex towards the anode, said filament and anode providing a path for electrons between the filament and the anode, a metal shield forming a focussing slot for the said filament with the edges of the shield adjacent the filament convex towards the anode, said shield being positioned with the axis of the anode passing through said slot to said filament, an element around the electron path, said element being cylindrical in form and defining the sole focussing device between the metal shield and the anode, and electric circuit means for providing an operating potential difference between filament and anode, for holding the element around the electron discharge path at a potential in the region of filament potential, and for holding the said metal shield at a negative potential relative to the filament.

12. An X-ray generator incorporating an X-ray tube having an anode, a slotted metal shield with the forward edges of the shield defining the slot convex towards the anode, a filament having an emission surface convex towards the anode and located within the slot in the shield, said s iield being positioned with the axis of the anode passing through the middle of said slot to said filament, an element around at least part of the space between the filament and anode, said element being cylindrical in form and defining the sole focussing device between the metal shield and the anode, and electric circuit means for providing an operating potential difference between cathode filament and anode, for holding the said element at a potential in the region of filament potential and for holding the said metal shield at a negative potential rela tive to the filament.

References Cited in the file of this patent UNITED STATES PATENTS 590,786 Bradley Sept. 28, 1897 2,347,424 Machlett Apr. 25, 1944 2,362,816 Harker Nov. 14, 1944 2,559,526 Van De Graaff July 3, 1951 2,952,790 Steen Sept. 13, 1960 

1. A CATHODE ASSEMBLY, FOR AN X-RAY TUBE OF THE KIND IN WHICH THE ELECTRON PATH FROM CATHODE TO ANODE LIES WITHIN AN ELEMENT WHICH IS MAINTAINED AT A POTENTIAL IN THE REGION OF CATHODE POTENTIAL, HAVING A FILAMENT HAVING AN EMISSION SURFACE CONVEX TOWARDS THE INTENDED DIRECTION OF ELECTRON FLOW AND A METAL SHIELD FORMING A FOCUSSING SLOT FOR THE SAID FILAMENT AND ARRANGED SO THAT IT CAN BE BIASSED AT A NEGATIVE POTENTIAL RELATIVE TO THE 